In hardcopy apparatus and particularly in apparatus handling media of big size, such as large format printers, printed media is outputted towards the outside of the printer by means of outputting means which may damage the quality of the printout. Conventional outputting means, in order to advance the printed media, employ elements for holding the media having a direct contact with the printed surface, which may cause ink smearing and other adverse affects on print appearance.
For instance, starwheels are employed in a number of apparatus for outputting printed media and may damage the printout with starwheel marks. Another drawback is the need to employ a mechanism or a structure to hold the starwheels themselves.
Conventionally, sheet holddown devices such as electrostatic or suction devices are employed only to reduce the effects of paper curl and cockle on dot placement during printing. In vacuum holddown devices, sheet flatness is maintained by providing suction between a support plate and the back surface of a sheet to be handled.
Cockle effect is the reluctance of the paper to bend smoothly. Instead it bends locally in a sharp fashion, creating permanent wrinkles.
Although conventional vacuum holddown devices are fairly effective in maintaining sheet flatness during printing, they have drawbacks. One drawback is the complexity of maintaining the same holddown force along the entire width of the medium while printing, i.e. in the direction of the printheads motion. This is due to the losses of air that the conventional devices allow, causing the medium to be subject to different forces, i.e. forcing the medium to rotate while it is advanced in the direction of the media motion.
Another drawback is that on one hand the maximum holddown force on a sheet is limited because of the necessity to maintain low frictional loads on transport devices which index the sheets. In conventional inkjet printers, such limitations can cause pen-to-sheet spacing distances to vary from swath to swath. Consequently, the holddown pressure at a localised area being printed may be insufficient to flatten cockles and other paper irregularities. On the other hand the vacuum required to eliminate cockle wrinkles in a printout would be so high that is normally unfeasible; in fact, high vacuum may suck the ink right through the paper and at the same time generate a lot of noise.
Applicant has then experimented that the employment of a vacuum holddown output unit may help media to be outputted without damaging the print appearance.